The invention relates generally to diagnostic imaging and, more particularly, to a method of CT perfusion imaging and an apparatus for implementing same.
Typically, in computed tomography (CT) imaging systems or scanners, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point that may be stationary or wobbled with respect to the rotating target during x-ray emission. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction. Typically, the scintillator is part of an array of scintillators.
The scintillator array typically extends in both circumferential (X) and axial (Z) directions of the CT scanner as is understood in the art. In a perfusion application, the axial coverage of the detector traditionally defines the axial region that may be studied. Thus for a 4 cm detector, as an example, a perfusion study may be conducted having a 4 cm organ positioned such that the source and detector rotate about the organ in a single axial location. However, for organs having greater than 4 cm in axial length (12-16 cm of coverage is desired for head perfusion studies, and 16 cm or more of coverage is desired for a liver perfusion study, as examples), the information obtained will be limited and important diagnostic imaging information may be missed. Coverage may be increased by using consecutive series of scans at contiguous axial locations, each with its own contrast injection. However, this tends to correspondingly increase both the contrast load and the radiation dose to the patient.
To reduce both the contrast load and radiation dose while increasing anatomical coverage, axial coverage of the detector may be increased by increasing the overall z-length of the array. However, because desired coverage may be 16 cm or more, this tends to increase the cost of the detector, hence the overall system, to prohibitive levels.
Alternatively, a subject may be positioned and data may be obtained during a single bolus injection at two axial locations in a volume axial shuttle (VAS) mode, or over an extended region in a volume helical shuttle (VHS). In VHS, while the table moves fore and aft, helical scanning data may be obtained throughout the range of motion and contrast uptake information and dynamic information can be obtained over time. During VHS the table may travel from a first Z location to a second Z location and dwell or idle at each first and second Z location while obtaining projection data throughout. Data thus acquired at a center of subject positions between first and second Z locations will be obtained at a generally uniform time interval as the table sweeps fore and aft. However, data obtained off-center will be non-uniform in time due to the shuttle motion of the table, with the most extreme non-uniformity occurring at the extremes of table motion.
In other words, data may be repeatedly sampled at the first Z location during dwell or idle of the table. The table then moves to the second Z location and dwells there, during which time data is repeatedly sampled at the second Z location. This results in a plurality of scans being obtained at the first Z location, and a gap in first Z location data then occurs while the table moves to the second Z location, dwells there and obtains projection data, and moves back to the first Z location. Likewise, a plurality of scans are obtained at the second Z location, and a gap in second Z location data then occurs while the table moves to the first Z location, dwells there, and moves back. The presence of these gaps, or sampling intervals, results in a non-uniform set of data that determines the accuracy of the perfusion map. To produce an acceptable perfusion map, the sampling interval needs to be less than approximately 3.2 seconds. This limitation of approximately 3.2 seconds thus limits the overall coverage in Z of VHS and limits the amount of stationary samples that may be taken during dwell or idle. Accordingly, if it is desired to conduct a perfusion study of an organ that is greater in z-length than can be obtained with a time limitation between first and second locations, then an additional perfusion study may be conducted—but at the expense of increased contrast load and radiation dose to the patient.
Therefore, it would be desirable to design an apparatus and method of VHS perfusion imaging having an increased ability to obtain perfusion data of a large organ without having to include multiple contrast injections.